Nowadays, more and more electronic devices use light-emitting diodes (LEDs) as a backlight module. In general electronic devices, such as televisions, computer displays can work at a two-dimensional (2D) mode or a three-dimensional (3D) mode. When the electronic device is worked at the 3D mode, it is need to scan a left-eye image and a right-eye image in sequence, and the duration to scan the left-eye image and the right-eye image are both the same. In a general 3D mode scanning method, one image is divided into a number of portions and then the portions are respectively be scanned, and the backlight module usually includes a number of LED strings, after one portion of the picture is scanned, one LED strings corresponding to the portion is turned on to emit light. Therefore, backlight controlling signals outputting to the LED strings corresponding to the portions of the picture are different signals which are delayed in sequence. However, in order to output the backlight controlling signals to the LED strings corresponding to different portions, there is need to prepare a number of signal lines, which causes the circuit is complex and the signal lines are easy to interfere each other.